Magnetic data storage and retrieval systems store and retrieve information on magnetic media. A magnetic head is supported relative to a magnetic media surface by a slider. During operation, the disc is rotated by a spindle motor which creates airflow along a storage interface surface (SIS) of the slider from a leading edge to a trailing edge of the slider. Airflow along the SIS of the slider creates a hydrodynamic lifting force so the head of the slider essentially flies above the surface of the magnetic media. The distance between the slider and the magnetic media is known as the fly height.
In a magnetic data storage and retrieval system, a magnetic head typically includes a writer portion for storing magnetically-encoded information on a magnetic media and a reader portion for retrieving the magnetically-encoded information from the magnetic media. To write data to the magnetic media, an electrical current is caused to flow through a conductive write coil to induce a magnetic field in a write pole. By reversing the direction of the current through the write coil, the polarity of the data written to the magnetic media is also reversed.
During operation of the magnetic data storage and retrieval system, the magnetic head is positioned in close proximity to the magnetic media. The distance between the magnetic head and the media is preferably small enough to allow for writing to and reading from the magnetic media with a large areal density, and great enough to prevent contact between the magnetic media and the magnetic head. Performance of the magnetic head depends primarily upon head-media spacing (HMS). High density recording preferably requires a small HMS and a low fly height. Prior to using each magnetic head, there are small variations in fly height that must be accounted for due to changing operating conditions and head-to-head variations.
As the need for data storage increases, the areal bit density of magnetic media also increases. In order to utilize the increased areal bit density of high density magnetic discs, it is necessary to reduce the fly height between the slider and the magnetic media surface. However, as fly height decreases, there is an increased possibility of unintentional contact between the magnetic head and the magnetic media. Extensive contact between the head and the magnetic media can damage the head and lead to loss of data. Thus, the fly clearance must be measured for each magnetic head by a controlled measurable non-destructive head-media contact so that the proper algorithm for operating the heater is used for each magnetic head.
In operation, the layers of the head, which include both metallic and insulating layers, all have different mechanical and chemical properties than the substrate. The differences in properties affect several aspects of the head, including pole tip protrusion of the metallic layers of the head with respect to the substrate at the SIS of the head. Two components of the pole tip protrusion effect exist, thermal pole tip protrusion and current-induced pole tip protrusion. Thermal pole tip protrusion arises from isothermal (global) temperature changes in the head during drive operation. Current-induced pole tip protrusion results from localized heating during application of currents to the write coil and the resultant heat dissipation into the surrounding components of the head. The pole tip protrusion must be accounted for when determining the proper fly height between the slider and the surface of the magnetic media.
The head-media contact is typically detected by a signal that changes sharply when the head mechanically contacts a lube layer of the magnetic media. For example, the signal could be ^PES (position error signal). In the ^PES method of detecting contact, when a head at skew contacts a lubricant layer on the media, it is dragged off-track more than when only flying. To compensate for this off-track drag force, a larger ^PES is generated by a positioning system to keep the head on track. Another method of detecting contact between the head and the magnetic media is acoustic emission (AE) detection. AE detection utilizes the ultrasound made when a head and magnetic media come into contact. To use ^PES or AE, the surface area of the head-media contact must be large enough so that when the thermally protruded magnetic head hits the lube layer of the magnetic media, the magnetic head component protruding most at the storage interface surface does not penetrate past the lube layer and start burnishing on the hard media surface, destroying the protective magnetic head layer.
Fly height control is particularly problematic in high-density magnetic data storage and retrieval systems that use perpendicular writers. In perpendicular writer designs, the return poles are positioned further away from the primary write pole when compared to longitudinal writer designs. During thermally induced contact, only a small region close to the primary write pole comes into contact with the magnetic media. Consequently, the contact area is much smaller for perpendicular writer designs. Both ^PES and AE depend on signals that are proportional to the surface area of the contact between the head and the magnetic media. Thus, the heads of perpendicular writers result in a smaller signal for use in contact detection by AE or ^PES. Additional factors, such as the speed at which the magnetic media revolves, the storage interface surface topology, and air bearing pressurization, can also reduce the contact signal.
Although the feature of the magnetic head that contacts the disc must have a large surface area to control clearance, due to process variations, the primary write pole does not always end up as the closest point to the disc for all magnetic heads. This is needed to minimize the HMS between the writer and the disc. Due to variations in the relative alignment between the primary write pole and the contact point, a significant percentage of magnetic heads have a recessed write pole, causing increased HMS of the primary write pole. Therefore, there is a need for both enhanced contact detection between the magnetic head and disc of a magnetic data storage and retrieval system and for improved alignment between the write pole and the large head-media contact surface.